1. [Field of the Invention]
The invention relates to efficient utilization of the non-simultaneous load advantage (NSLA)(heretoafter referred to as "NSLA") of a time-compressed multiplexed (TCM) signal, which is a modulated signal used in a time-division method for radio channels applied to radio communications between mobile bodies. More particularly, it is directed to a communication method which improves frequency utilization efficiency and thus achieves economic mobile radio communications in the following way. A radio channel is given, and while one of a plurality of movable radio units within a service area is communicating with its counterpart radio base station by establishing a radio communication line on the given radio channel, another mobile radio unit initiates communication with another radio base station using the same radio channel. Under such condition, interference with the communication between the mobile radio units and the radio base stations to be caused due to frequency utilization efficiency considerations or radio wave propagation characteristics is obviated. Also, separate communication signals are impressed on a band of low frequencies other than the frequencies carrying the time-division multiplexed signals, and the NSLA of these separate communication signals are utilized to reduce transmission power.
2. [Related Art]
Systems using time-division time-compressed multiplexed (TCM) signals for small-zone voice signal communications between mobile bodies are discussed in the following references.
Reference 1: S. Ito "A Study of a Portable Telephone System--A Proposal of Time-Division Time-Compressed FM System," IECE Technical Report, RCS 89-11, July, 1989; and
Reference 2: S. Ito "A Study of a Portable Telephone System--A Theoretic Study of Time-Division Time-Compressed FM System," IECE Technical Report, RCS 89-39, October, 1989.
Specifically, reference 1 reports an exemplary system which includes: a radio reception circuit having a reception mixer; a radio transmission circuit having a transmission mixer; and a switching circuit. The radio reception circuit and the radio transmission circuit communicate with each other and are included in each of a mobile radio unit and a radio base station to intermittently receive and transmit signals therebetween. Each circuit divides a transmission signal (base band signal) by a predetermined time interval; stores the divided signals in a storage circuit; reads each stored signal into a predetermined time slot at a reading speed which is n times a storing speed; and angle-modulates or amplitude-modulates a carrier by the modulating signal loaded in the time slot. The switching circuit is arranged in both a frequency synthesizer (heretoafter referred to as "synthesizer") to be applied to the reception mixer of the radio reception circuit and a synthesizer to be applied to the transmission mixer of the radio transmission circuit. In this exemplary system a transmitted original base band signal can be reproduced in the following way. The output of each synthesizer is applied intermittently, and not only transmission and reception synchronize with these intermittent outputs, but also a pair of radio base station and mobile radio unit synchronize with each other under the intermittent transmission and reception. To access only the signal loaded in the predetermined time slot, the reception side receives the transmission signal by connecting and disconnecting the radio reception circuit; storing the demodulated signal in a storage circuit; and reading the stored signal at a low reading speed which is 1/n times a storing speed.
Reference 2 discusses interference between adjacent radio channels and co-channel interference, which are problems to be handled in a small-zone TCM-FM system, and suggests the possibility that a satisfactory system will be implemented through proper design of system parameters.
NSLAs of so-called frequency-division multiplexed (FDM) signals, in which voice signals are frequency-converted and multiplexed so that they do not overlap on the frequency axis, are discussed, e.g., in the following references.
Reference 3: B. D. Holbrook, J. T. Dixon: "Load Rating Theory for Multichannel Amplifiers" BSTJ, 18, Oct. 1939; and
Reference 4: C. B. Feldman, et. al., "Band Width and Transmission Performance" BSTJ, July, 1949, pp. 490 to 595.
FIG. 13 is prepared from FIG. 7 of the above reference 3, while FIG. 14 is cited from page 495 of the above reference 4 which indicates that NSLAs substantially equal to those shown in FIG. 13 can be obtained.
Reasons why NSLAs are obtained and their applications to angle modulation will be briefly described hereunder.
The level of a communication line which is busy with an active telephone signal varies depending on the person, sex, and length of a subscriber line, and there is always an interval between words in a continuous speech by a single person. While one party is talking, the other does not talk, thereby transmitting no signal in one direction. No speech is made while the communication line is being switched and connected. Thus, individual signal levels are diversified, and to obtain a signal having these different levels synthesized is not easy. But to clarify this point is very important in preparing repeater lines capable of maintaining levels of distortion, crosstalk, quasi-crosstalk, and noise within predetermined tolerances. Thus many people have been involved in this study.
The level of a signal in a carrier-suppressed FDM system (SS: a system to which a single side band is applied) is a synthesis of these voices. It is very unlikely that the individual voices overlap one another simultaneously. Thus, as long as the number of communication lines N is small, the impact of each individual voice on the synthetic signal is direct due to its large fluctuation. However, its impact becomes less direct with increasing degree of multiplexing to be averaged in terms of probability. Thus, the peak of the synthetic signal increases very slowly with an increase in the number of communication lines. This tendency has been statistically demonstrated with respect to telephone signals in the United States by B. D. Holbrook and J. T. Dixon (in reference 3). According to their study, fluctuations in the power of a sinusoidal wave having a peak value equal to that of a multiplexed signal are as shown in FIG. 13. To demonstrate how small the increase in peak of the multiplexed signal is, such increase is compared with a sum of the peak voltages of individual signals. The result is NSLAs shown in FIG. 13. That is, e.g., a 960-multichannel system has a peak voltage equal to that obtained when all signals are fully loaded to 6 communication lines simultaneously and no signals are loaded to 954 communication lines.
In an SS-FM system, fluctuations in voltage of the synthetic signal result in a frequency deviation. Therefore, if it is assumed that the number of multiplexed communication lines N is increased with the peak frequency deviation of the synthetic signal defined to be a predetermined value, the modulation index per communication line can be increased by each NSLA shown in FIG. 13 compared with that in the case where the voltages of all the communication signals are summed up. This contributes to improving its signal to noise ratio (S/N) commensurate with the increase in the modulation index, compared with the S/N ratio given at the time the peak frequency deviation is defined to be an arbitrary value.
The exemplary system configurations proposed in references 1 and 2 include no disclosure as to the presence of NSLAs of TCM signals transmitted from a radio base station to a multiple of mobile radio units. Thus, they do not take advantage of the NSLAs.
Thus, there exists a problem to be overcome; i.e., the problem of not being able to achieve a number of advantages which could have been achieved if the NSLAs had been analyzed therein. Such advantages include: reduction in transmission output level to be achieved by increasing the frequency modulation factor; ease with which to design TCM signal amplifiers; implementation of economical amplifiers by increasing the operation level setting range; and system economization by relaxing the rating restrictions of mixers, resistors, and capacitors.
The exemplary system configurations disclosed in references 3 and 4 introduce the multiplexed load gain in the so-called FDM signal in which voice signals are frequency-converted and multiplexed so that they do not overlap on the frequency axis. However, these systems are not applicable to TCM signals, nor do they clearly refer to the presence of multiplexed load gains in the TCM signals. Thus, there also exists a problem to be overcome; i.e., the problem of not being able to achieve a number of advantages (the same advantages mentioned with respect to the problem related to the references 1 and 2) which could have been achieved in system design if the presence of the multiplexed load gains in the TCM signals had been clearly indicated in these references 3 and 4.
Further, time-compressed multiplexing (TCM), in which original signals are time-compressed, produce an unused band of low frequencies. Thus, effective use of such low-frequency band remains as another problem to be overcome.